Drogos, what do you have the boost filter set to? (menu option 1)
In the past there has been a lot of discussion about transient boost spikes and VBG1 sampling methods producing higher readings. We added boost filtering into the unit as an option to knock some of those transients down. So if your boost filtering is set higher it will reduce the peak values reported.
If you desire a reading as close to "real" pressure as you can get - I recommend the setting of "Low" boost filter.
A bunch of the AU guys were looking into this at one point:
http://forum.liberty.asn.au/viewtopic.php?f=56&t=20436&start=60
They concluded that the VBG1 was more accurate on transients then their other form of measurement.
The ECU and the VBG1 are inherently sampling on different clocks and at different rates. So depending on how the sampling points line up how fast the log actually reports boost, its entirely likely the whole difference is just sampling related.
http://www.ni.com/cms/images/devzone/tut/sampleclk.JPG
That's correct - The old units had no boost filtering so peak transient values were latched as peaks and displayed after a pull, sometime up to 1-2 higher than logs were seeing.
Not likely - The start up procedure is pretty straight forward. The VGB1 takes somthing like 64 samples of the the boost line pressure (assumed to be ATM pressure) then averages the value and stores that as the baseline ATM value to calculate boost psi and vac. If you are using the approved fuse location you should be all set. People who have used the wrong location (improper power sequencing) saw wild results, if I remember correctly they were seeing almost no vac and higher boost by a +8 PSI.
All that being said if you think your unit is acting funny you can always PM me or email me and I can take a look at it. I know people depend on their gauges reading accurately so I take the concerns seriously.